In recent years, with an aim toward effective utilization of energy for greater environmental conservation and reduced usage of resources, a great deal of attention is being directed to power smoothing systems for wind power generation or overnight charging electric power storage systems, household dispersive power storage systems based on solar power generation technology, and power storage systems for electric vehicles and the like.
The number one requirement for cells used in such power storage systems is high energy density. The development of lithium ion batteries is advancing at a rapid pace, as an effective strategy for cells with high energy density that can meet this requirement.
The second requirement is a high output characteristic. A high power discharge characteristic is required for a power storage system during acceleration in, for example, a combination of a high efficiency engine and a power storage system (such as in a hybrid electric vehicle), or a combination of a fuel cell and a power storage system (such as in a fuel cell electric vehicle).
Electrical double layer capacitors and nickel-metal hydride batteries are currently under development as high output power storage devices.
Electrical double layer capacitors that employ activated carbon in the electrodes have output characteristics of about 0.5 to 1 kW/L. Such electrical double layer capacitors have high durability (cycle characteristics and high-temperature storage characteristics), and have been considered optimal devices in fields where the high output mentioned above is required. However, their energy densities are no greater than about 1 to 5 Wh/L. A need therefore exists for even higher energy density.
On the other hand, nickel-metal hydride batteries employed in existing hybrid electric vehicles exhibit high output equivalent to electrical double layer capacitors, and have energy densities of about 160 Wh/L. Still, research is being actively pursued toward further increasing their energy density and output, and increasing their durability.
Research is also advancing toward increased outputs for lithium ion batteries as well. For example, lithium ion batteries are being developed that yield high output exceeding 3 kW/L at 50% depth of discharge (a value representing the state of the percentage of discharge of the service capacity of a power storage element). However, the energy density is 100 Wh/L or lower, and the design is such that the high energy density, which is the major feature of a lithium ion battery, is reduced. The durability (especially cycle characteristic and high-temperature storage characteristic) is inferior to that of an electrical double layer capacitor. In order to provide practical durability, therefore, these are used with a depth of discharge in a narrower range than 0 to 100%. Because the usable capacity is even lower, research is actively being pursued toward further increasing the durability of lithium ion batteries.
There is a strong demand for implementation of power storage elements exhibiting high energy density, high output characteristics and durability, as mentioned above. Nevertheless, the existing power storage elements mentioned above have their advantages and disadvantages. New power storage elements are therefore desired that can meet these technical requirements.
Promising candidates are power storage elements known as lithium ion capacitors, which are being actively developed in recent years.
The energy of a capacitor is represented as ½·C·V2 (where C is electrostatic capacitance and V is voltage).
A lithium ion capacitor is a type of power storage element using a nonaqueous electrolytic solution comprising a lithium salt (or, “nonaqueous lithium power storage element”), wherein charge/discharge is accomplished by: non-Faraday reaction by adsorption/desorption of anions similar to an electrical double layer capacitor at about 3 V or higher, at the positive electrode; and Faraday reaction by intercalation/release of lithium ions similar to a lithium ion battery, at the negative electrode.
To summarize these electrode materials and their characteristics: when charge/discharge is carried out using a material such as activated carbon as an electrode, by adsorption and desorption of ions on the activated carbon surface (non-Faraday reaction), it is possible to obtain high output and high durability, but with lower energy density (for example, one-fold). When charge/discharge is carried out by Faraday reaction using an oxide or carbon material as the electrode, the energy density is higher (for example, 10 times that of non-Faraday reaction using activated carbon), but then durability and output characteristic become issues.
Electrical double layer capacitors that combine these electrode materials employ activated carbon as the positive electrode and negative electrode (energy density: one-old), and carry out charge/discharge by non-Faraday reaction at both the positive and negative electrodes, and are characterized by having high output and high durability, but also low energy density (positive electrode: one-fold×negative electrode: one-fold=1).
Lithium ion secondary batteries use a lithium transition metal oxide for the positive electrode (energy density: 10×) and a carbon material (energy density: 10-fold) for the negative electrode, carrying out charge/discharge by Faraday reaction at both the positive and negative electrodes, and have high energy density (positive electrode: 10-fold×negative electrode: 10-fold=100), but have issues in terms of output characteristic and durability. The depth of discharge must be restricted in order to satisfy the high durability required for hybrid electric vehicles, and with lithium ion secondary batteries only 10 to 50% of the energy can be utilized.
A lithium ion capacitor is a new type of asymmetric capacitor that employs activated carbon (energy density: one-fold) for the positive electrode and a carbon material (energy density: 10-fold) for the negative electrode, and it is characterized by carrying out charge/discharge by non-Faraday reaction at the positive electrode and Faraday reaction at the negative electrode, and thus having the characteristics of both an electrical double layer capacitor and a lithium ion secondary battery. A lithium ion capacitor exhibits high output and high durability, while also having high energy density (positive electrode: one-fold×negative electrode: 10-old=10) and requiring no restrictions on depth of discharge as with a lithium ion secondary battery.
As explained above, the potential uses of nonaqueous lithium power storage elements such as lithium ion batteries and lithium ion capacitors include automobiles and the like, but as their applications continue to increase in the future there will be greater demand for an even higher level of improved energy density, output characteristics and durability.
In light of this background, as a measure for improving the output characteristic and cycle durability of a lithium ion battery, it has been proposed in PTLs 1 and 2 to appropriately specify the conductive filler amount, voids and pore sizes in the positive electrode active material layer of the positive electrode, in order to form a satisfactory conductive pathway in the positive electrode active material layer, increase the lithium ion conductivity and ensure electrolytic solution retentivity.
However, the technique described in PTL 1 is particularly dependent on the pores formed by the gaps between the conductive filler in the positive electrode active material layer, and despite retentivity of the electrolytic solution in the pores, linkage between the conductive filler is easily broken, and therefore the input/output characteristic has had room for improvement. Moreover, while the technique described in PTL 2 ensures the void percentage or pore size and increases the lithium ion conductivity, it is also associated with lower positive electrode bulk density and has potentially resulted in reduced energy density.
In PTL 3 there is proposed a lithium ion secondary battery using a positive electrode containing lithium carbonate in the positive electrode, and having a current shielding mechanism that operates in response to increased internal pressure in the battery. In PTL 4 there is proposed a lithium ion secondary battery employing a lithium complex oxide such as lithium manganate as the positive electrode, and with reduced elution of manganese by including lithium carbonate in the positive electrode. In PTL 5 there is proposed a method of causing restoration of the capacitance of a deteriorated power storage element by oxidizing different lithium compounds as coated oxides at the positive electrode.
However, the methods described in PTLs 3 to 5 are associated with the problems of increased resistance due to decomposition of residual lithium compounds in the positive electrode, and reduced energy density, and therefore further room for improvement exists in terms of the high load charge/discharge characteristic.
On the other hand, PTL 6 discloses a power storage device wherein the negative electrode active material used is mesoporous graphite of which the volume of mesopores with pore diameters of 100 Å to 400 Å occupies 25% to 85% of the total mesopore volume, and wherein a satisfactory output characteristic is exhibited at room temperature and at low temperature.
However, PTL 6 contains no description regarding the pore volume, specific surface area. and distribution thereof in the negative electrode active material layer. Research by the present inventors has shown that by merely adjusting the pore volume, specific surface area and their distribution in the negative electrode active material alone, it is difficult to obtain a sufficient input/output characteristic and high load charge/discharge cycle characteristic, in a nonaqueous lithium power storage element using the material. The pore volume, specific surface area and distribution thereof in a negative electrode active material layer are significantly affected by the types of negative electrode active material, conductive filler and binder, as well as their weight ratios in the negative electrode active material layer, or the amount of coverage of the coating or accumulation by reductive decomposition of the nonaqueous electrolytic solution at the negative electrode active material layer. Thus, it was found that the pore volume, specific surface area and their distribution in the negative electrode active material layer have an effect on the input/output characteristic and high load charge/discharge cycle characteristic, in a nonaqueous lithium power storage element using the material.